Circuit board through-holes are commonly plated or filled with conductive material in order to provide for the transfer of electrical signals from one surface of the circuit board layer to a second surface of the circuit board layer.
Filled through-holes are typically through-holes which receive contacts or pins of some element external to the circuit board. The through-holes typically are filled manually with a molten conductive material, to both physically and electrically connect the external element to the circuit board. It is also known to form a plug in the through-holes by a method which includes filling the through-holes with a first layer of conductive material, curing this layer, planarizing the circuit board layer to remove the conductive material cured to the surface of the circuit board layer, filling the through-holes with a second conductive material, and curing this second material. Both of these methods are time-consuming and expensive. Furthermore, not only does the latter method require several filling steps, but it also involves a substantial alteration of the surface of the circuit board layer in order to remove the conductive material that is cured onto the surface of the circuit board layer.
Plated through-holes generally involve electroless copper plating which is expensive and coats only the inside surface of the through-holes. It is known to form a thin meniscus of non-conductive or dielectric material within a via plated with conductive material using a screen without a stencil. This typically is done to allow vacuum hold down of circuit boards for testing, or to prevent assembly materials from propagating from one side of the circuit board to the other, but may be done for other reasons as well. However, the meniscus formed is susceptible to corrosion, thus decreasing the yield of acceptable circuit boards and increasing their cost.
Furthermore, methods of forming plugs in circuit board layers which result in significant alteration of the surfaces of circuit board layer is a disadvantage, particularly if it is desirable to print the circuit board prior to plug formation.
A problem encountered with sequential lamination, as well as lamination in general, is resin loss during lamination. Resin loss is problematic as lost resin present on the surface of the laminate will cure to the outer surfaces of the laminate. Further, resin loss may result in insufficient lamination of the layers. Also, loss of resin into vias may result in a dimpled laminate surface over the via, due to the loss of resin in that area.
In sequential lamination of printed circuit board subassemblies, resin loss typically is reduced by placing rubber pads against the outer layers of the subassemblies during lamination in order to stop resin flow to the outer surfaces of the subassemblies. One disadvantage to this approach is that resin still flows into the vias and, in some cases, results in unsatisfactory bonding between the subassemblies.
Therefore, there exists a need in the art for a method and apparatus to quickly and efficiently form plugs in circuit board through-holes at an acceptable yield and cost. There is a further need for a method of forming plugs in circuit board through-holes without significantly altering either surface of the circuit board layer. Finally, there exists a need in the art for a method to provide a plug with minimal voids formed therein.